Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Here the strongest acid is (a) \(\mathrm{CH}_{3}-\mathrm{COOH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{COOH}\) (c) \(\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{COOH}\) (d) \(\mathrm{Br}-\mathrm{CH}_{2}-\mathrm{COOH}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{COOH}\) is the strongest acid.

Step 1: Identify the Structure

Recognize that each option represents a carboxylic acid and it's important to note the differences in their structures. (a) Acetic acid, (b) Propionic acid, (c) Chloroacetic acid, and (d) Bromoacetic acid are what we have here.

Step 2: Consider the Influence of Substituents

Understand that the presence of electronegative atoms (like Cl and Br) near the carboxyl group can influence acidity. Halogens like Cl and Br withdraw electron density from the carboxyl group, stabilizing the conjugate base which enhances acidity.

Step 3: Compare Electronegativities

Chlorine (Cl) is more electronegative than Bromine (Br), which means it will have a greater electron-withdrawing effect. This means option (c) \(\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{COOH}\) will stabilize its conjugate base more than option (d).

Step 4: Predict the Strongest Acid

Based on the inductive effect of the substituents, \(\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{COOH}\) with the strongest electron-withdrawing group (Cl) will be the strongest acid. Compare this with options a and b where no halogens are present.

Key Concepts

Electronegative Substituents

In carboxylic acids, the presence of electronegative substituents plays a crucial role in determining the acidity. An electronegative group, such as halogens (Cl or Br), has the ability to attract electron density towards itself. This makes the molecule more polarized. When these atoms are close to the carboxyl group (\(-COOH\)), they can enhance the overall acidity of the acid. Here's why:

• Electronegative atoms increase the polarization within the molecule, making it easier for the hydrogen in the \(-OH\) group to dissociate as a proton (\(H^+\)).
• The more electronegative the atom, like Cl as compared to Br, the greater its ability to enhance acid strength.

This is because the more electronegative chlorine atom pulls electron density more effectively than bromine, making it easier for the acid to release a proton. This means chloroacetic acid is stronger than bromoacetic acid given the same structure, emphasizing the role of electronegative substituents in acid strength.

Inductive Effect

The inductive effect is a critical concept to grasp when understanding acidity in carboxylic acids. It refers to the transmission of charge through a chain of atoms in a molecule. When an electronegative atom, such as chlorine in chloroacetic acid, is present, it draws electron density away from other parts of the molecule. This movement of electron density down the molecule enhances the acidity of the carboxylic acid. Here's how:

• The inductive effect stabilizes the negative charge on the conjugate base formed after the acid releases its proton.
• The stronger the electronegative atom, the more pronounced the inductive effect, leading to a stronger acid.

Thus, a stronger inductive effect due to the presence of chlorine ensures that chloroacetic acid can stabilize its conjugate base more than bromoacetic acid or acetic acid, resulting in a stronger acid overall.

Stability of Conjugate Base

The stability of the conjugate base is an essential factor in determining the acidity of carboxylic acids. When a carboxylic acid donates a proton, it forms a conjugate base that must be stable to maintain the acidity. Here's why stability is important:

• Stable conjugate bases are less likely to revert to the acid form, promoting the dissociation of the acid.
• Electronegative substituents enhance this stability by dispersing the negative charge over a larger area.

For instance, in chloroacetic acid, chlorine's electron-withdrawing ability helps to stabilize the conjugate base by distributing the negative charge away from the most electronegative atom, increasing overall stability. Hence, the more stable the conjugate base, like that of chloroacetic acid, the stronger the acid will be.